Metal oxide nanostructured surfaces

ABSTRACT

Embodiments of nanostructures comprising metal oxide and methods for forming the nanostructure on surfaces are disclosed. In certain embodiments, the nanostructures can be formed on a substrate made of a nickel titanium alloy, resulting in a nanostructure containing both titanium oxide and nickel oxide. The nanostructure can include a lattice layer disposed on top of a nanotube layer. The distal surface of the lattice layer can have a titanium oxide to nickel oxide ratio of greater than 10:1, or about 17:1, resulting in a nanostructure that promotes human endothelial cell migration and proliferation at the interface between the lattice layer and human cells or tissue. The nanostructure may be formed on the outer surface of an implantable medical device, such a stent or an orthopedic implant (e.g. knee implant, bone screw, or bone staple).

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

FIELD

Embodiments of the present invention relate to nanostructures includingmetal oxide and methods of making and using the same.

BACKGROUND

Surface modifications have been explored to provide beneficialcharacteristics to a variety of devices. One of the methods that hasbeen explored for preparing such surface modifications iselectrochemical anodization. In electrochemical anodization, the surfacebeing modified forms an anode electrode. The anode is generally thenplaced into electrical contact with at least one cathode through anelectrolyte solution. A voltage is then applied across the anode andcathode for a period of time. Anodization often results in undesirableand uncontrollable amorphous material accumulating on the surface of theanode. Moreover, the resulting material may contain too high of aconcentration of cytotoxic elements, such as nickel, rendering theresulting surface unusable for many potential applications. Accordingly,there is a need for reliable methods of preparing surface modificationswith desirable structures and elemental compositions.

SUMMARY

The present invention relates to nanostructures comprising metal oxideand methods of forming the same on the surface of substrates. Thesubstrates may be titanium or any titanium alloy, such as a nickeltitanium alloy. The metal oxide nanostructures may have a lattice layerdisposed on top of a nanotube layer disposed on top of the surface of asubstrate. The lattice and nanotube layers may be formed during ananodization process. The distal surface of the lattice layer may or maynot be enriched in titanium. Some embodiments of the present metal oxidenanostructures and methods of forming the nanostructures on surfaceshave several features, no single one of which is solely responsible fortheir desirable attributes.

While some embodiments of the resulting metal oxide nanostructuredsurfaces provide numerous advantages over other known metal oxidelayers, they may be particularly beneficial when used with implantablemedical devices because they may promote healing by enhancing theintegration of the medical device with the surrounding biologicaltissue. An example is the promotion of endothelial cell migration andproliferation and the inhibition of smooth muscle cell migration andproliferation in the cardiovascular and systemic vascular systems. Witha medical device such as a stent, for example, some embodiments maypromote the growth of a confluent endothelial layer while inhibiting thegrowth of the neointima, thus reducing restenosis and leading tolong-term patency at the site of implantation. The medical device canalso be an orthopedic implant, such as a knee implant, bone screw, orbone staple, such as those used for hand and foot bone fragmentsosteotomy fixation and joint arthrodesis.

In one embodiment, the nanostructure comprises two layers: a first layerof tubular structures, and a second layer comprising a lattice. Thenanostructure can be arranged such that the first layer is situatedbetween a substrate (e.g., a metal article) and the second layer. Thesubstrate can be any suitable metal article, including an implantablemedical device such as a stent. In some embodiments, the surface of themetal article comprises a titanium alloy, such as a nickel titaniumalloy. The first layer can be formed at least in part from the surfaceof the substrate via anodization. The tubular structures comprise metaloxide(s). The tubular structures may or may not further comprise carbon,fluoride, or other elements. In some embodiments, the tubular structurescomprise titanium oxide and nickel oxide. The composition of the tubularstructures depends in part on the composition of the surface ofsubstrate from which they are formed. The nanostructure can be formed onsubstantially (e.g. at least about 90%) all of the entire outer surfaceof a substrate, or it can be formed on a single or multiple discretelocations on the surface of a substrate.

Each of the tubular structures of the first layer has a cylindrical wallwith an inner surface, a top surface, and an outer surface, thecylindrical wall defining a lumen therein. The geometry of a tubularstructure generally defines an axis that further generally defines adirection of the tubular structure. If a tubular structure were acircular cylinder, then the axis is the locus of the centers of thecircles defined by the tubular structure. With tubular structures havinga polygonal or irregular cross section, the axis generally follows thecenter of the tubular structure. Collectively, the top surfaces of thetubular structures define a distal surface of the first layer. In someembodiments, the tubular structures can be aligned generallyperpendicular to the substrate and generally parallel to one another(e.g. arranged vertically compared to a horizontal substrate). In otherembodiments, at least some of the tubular structures can be formed at anangle other than 90° relative to the substrate. In some embodiments, thetubular structures are immediately adjacent to one another, such thatthere is no observable space between the outer surface of a cylindricalwall of one tubular structure and the outer surface(s) of thecylindrical wall(s) of the adjacent tubular structure(s). In someembodiments, the outer surface of the cylindrical wall of one tubularstructure is in direct contact with the outer surface(s) of thecylindrical wall(s) of the adjacent tubular structure(s). In someembodiments, the outer surface of the cylindrical wall of one tubularstructure is integrally connected with the outer surface(s) of thecylindrical wall(s) of the adjacent tubular structure(s). In otherembodiments, the tubular structures can be arranged such that there isspace between the cylindrical wall of one tubular structure and thecylindrical wall of the adjacent tubular structures. In someembodiments, tubular structures are not cylindrical and can define avariable cross-sectional area along the axial direction. In someembodiments, tubular structures can define polygonal or irregularopenings that are approximately or generally perpendicular to the axialdirection. The openings can be characterized by major and minor axes anda hydraulic diameter. A hydraulic diameter is a ratio of four times anarea divided by a perimeter of a given opening.

The second layer comprises a lattice disposed on top of the distalsurface of the first layer. The lattice comprises metal oxide(s). Thelattice may or may not further comprise carbon, fluoride, or otherelements. This second layer can be in contact with or integrallyconnected to the distal surface of the first layer (e.g. the top surfaceof cylindrical walls of the plurality of tubular structures). Comparedto the discrete tubular structures of the first layer, the second layergenerally comprises a single integrated structure. In some embodiments,the distal surface of the second layer is significantly free of anyamorphous material or particulate with dimensions greater than about 30nm. In some embodiments, the lattice comprises metal oxides includingbut not limited to titanium oxide and nickel oxide. In some embodiments,the ratio of titanium oxide to nickel oxide on the distal surface (e.g.the top about 10 nm) of the second layer is greater than about 10:1, orabout 17:1.

Methods of forming a nanostructure comprising metal oxide on the surfaceof a substrate are also disclosed herein. In one embodiment, the methodcomprises placing an anode and at least one cathode in electricalcontact through a first electrolyte solution, applying a first voltageacross the anode and cathode(s) through the first electrolyte solutionfor a first time period, modifying or replacing the first electrolytesolution to provide a second electrolyte solution containing an acid,and applying a second voltage across the anode and cathode(s) throughthe second electrolyte solution for a second time period. In someembodiments, the anode and cathode(s) are optionally provided. In someembodiments, the first electrolyte does not contain an acid or containsa very low amount of acid. The first electrolyte solution may bemodified by adding an acid, resulting in the second electrolytesolution. Alternatively, the first electrolyte solution may be removedand replaced with a second electrolyte solution that contains an acid.In some embodiments, the first electrolyte solution may be modified bylowering the pH or increasing the acid concentration to achieve thesecond electrolyte solution. In some embodiments, the substrate is notmodified to remove any portion of the oxide structure(s) formed duringthe first time period prior to the second time period. The first andsecond voltages can be the same or different. The first and second timeperiods can be the same or different. The temperature of the electrolytesolution in the first and second time periods can be the same ordifferent.

In another embodiment, the method comprises placing an anode and atleast one cathode in electrical contact through a first electrolytesolution, applying a first voltage across the anode and cathode(s)through the first electrolyte solution for a first time period, removingat least part of the oxide layer formed on the surface of the anodeduring the first time period, and applying a second voltage across theanode and cathode(s) through the second electrolyte solution for asecond time period. In some embodiments, the anode and cathode(s) areoptionally provided. The first and second voltages can be the same ordifferent. The first and second time periods can be the same ordifferent. The temperature of the electrolyte solution in the first andsecond time periods can be the same or different.

In another embodiment, the method comprises placing an anode and atleast one cathode in electrical contact through a first electrolytesolution, applying a first voltage across the anode and cathode(s)through the first electrolyte solution for a first time period, removingat least part of the oxide layer formed on the surface of the anodeduring the first time period, modifying or replacing the firstelectrolyte solution to provide a second electrolyte solution containingan acid, and applying a second voltage across the anode and cathode(s)through the second electrolyte solution for a second time period. Insome embodiments, the anode and cathode(s) are optionally provided. Thefirst electrolyte solution may be modified by adding an acid.Alternatively, the first electrolyte solution may be removed andreplaced with a second electrolyte solution that contains an acid. Insome embodiments, the first electrolyte solution may be modified bylowering the pH or increasing the acid concentration to achieve thesecond electrolyte solution. The first and second voltages can be thesame or different. The first and second time periods can be the same ordifferent. The temperature of the electrolyte solution in the first andsecond time periods can be the same or different.

In some embodiments of the methods discussed above, the anode can be animplantable medical device, including but not limited to a stent. Themedical device can also be an orthopedic implant, such as a kneeimplant, bone screw, or bone staple, such as those used for hand andfoot bone fragments osteotomy fixation and joint arthrodesis. Thecathode can be platinum, iron, stainless steel, graphite, or any otherconductive material.

Methods of forming a biocompatible nanostructure on the surface of animplantable medical device are also disclosed herein. In someembodiments, the methods comprise placing an anode and cathode inelectrical contact through a first electrolyte solution containingethylene glycol, water, and ammonium fluoride, applying a first voltageacross the anode and cathode through the first electrolyte solution fora first time period, modifying or replacing the first electrolytesolution resulting in a second electrolyte solution containing an acid,and applying a second voltage across the anode and cathode through thesecond electrolyte solution for a second time period. In someembodiments, an implantable medical device is optionally provided as theanode. In some embodiments, the cathode is optionally provided. In someembodiments, the first electrolyte solution does not contain an acid. Insome embodiments, the first electrolyte solution can be modified byadding an acid, including but not limited to sulfuric acid or hydroiodicacid. In other embodiments, the first electrolyte solution can beremoved and replaced with a second electrolyte solution that contains anacid, including but not limited to sulfuric acid or hydroiodic acid. Thefirst and second voltages can be the same or different. The first andsecond time periods can be the same or different. The temperature of theelectrolyte solution in the first and second time periods can be thesame or different. In some embodiments, the first and second voltagesare about 25V, the first and second time periods are about 5 minutes,and the temperature of the electrolyte solution in the first and secondtime periods is about 30 C. In some embodiments, the oxide materialformed during the first time period is not removed or modified prior tothe second time period.

These, as well as other components, steps, features, objects, benefits,and advantages, will now become clear from a review of the followingdetailed description of illustrative embodiments, the accompanyingdrawings, and the claims.

BRIEF DESCRIPTION OF DRAWINGS

The drawings are of illustrative embodiments. They do not illustrate allembodiments. Other embodiments may be used in addition or instead.Details that may be apparent or unnecessary may be omitted to save spaceor for more effective illustration. Some embodiments may be practicedwith additional components or steps and/or without all of the componentsor steps that are illustrated. When the same numeral appears indifferent drawings, it refers to the same or like components or steps.

FIG. 1 shows a schematic representation of a side view one embodiment ofa metal oxide nanostructure.

FIG. 2 shows a schematic representation of a partial side and topprofile view of the metal oxide nanostructure of FIG. 1 .

FIG. 3 shows a schematic representation of a top view of the latticelayer of the metal oxide nanostructure of FIG. 1 .

FIG. 4 shows a cross-sectional view of the metal oxide nanostructure ofFIG. 1 as viewed along plane 10-10.

FIG. 5 shows a schematic representation of a top view of a singletubular structure from the nanotube layer of the metal oxidenanostructure of FIG. 1 .

FIG. 6 shows a schematic representation of a top view of one embodimentof the lattice layer in FIG. 1 .

FIG. 7 shows a schematic representation of a top view of anotherembodiment of the lattice layer in FIG. 1 .

FIG. 8 shows a process flow diagram for one embodiment of a method offorming metal oxide nanostructures.

FIG. 9 shows a process flow diagram for another embodiment of a methodof forming metal oxide nanostructures.

FIG. 10 shows a process flow diagram for another embodiment of a methodof forming metal oxide nanostructures.

FIG. 11 shows a process flow diagram for one embodiment of a method offorming metal oxide nanostructures on an implantable medical device.

FIG. 12 shows a process flow diagram for another embodiment of a methodof forming metal oxide nanostructures.

FIG. 13 shows a process flow diagram for another embodiment of a methodof forming metal oxide nanostructures.

FIG. 14 shows a representative current profile resulting from performingthe method described in Example 1.

FIG. 15 shows a representative scanning electron microscope (SEM) imageof a metal oxide nanostructured surface resulting from the methoddescribed in Example 1.

FIG. 16 shows a representative SEM image of a metal oxide nanostructuredsurface resulting from the method described in Example 2.

FIG. 17 shows a schematic representation of part of the experimentalsetup described in Example 5.

FIGS. 18A-D show plots of the data resulting from the experimentsdescribed in Example 5. FIG. 18A shows a plot of the total cell count,based on nuclear stain, of primary human aortic endothelial cells on thesurface of the nanostructured nitinol coupons compared to the controlcoupons for each of the time points (24 hours, 48 hours, 72 hours, 96hours). FIG. 18B shows a plot of the total cell area on the surface ofthe nanostructured nitinol coupons compared to the control coupons foreach of the time points (24 hours, 48 hours, 72 hours, 96 hours). FIG.18C shows a plot of the nuclei count per substrate for thenanostructured nitinol coupons compared to the control coupons for eachof the time points (24 hours, 48 hours, 72 hours, 96 hours). FIG. 18Dshows a plot of the total cell area per substrate for the nanostructurednitinol coupons compared to the control coupons for each of the timepoints (24 hours, 48 hours, 72 hours, 96 hours).

FIGS. 19A-D show plots of the data resulting from the experimentsdescribed in Example 6. FIG. 19A shows a plot of the total cell count,based on nuclear stain, of human aortic smooth muscle cells on thesurface of the nanostructured nitinol coupons compared to the controlcoupons for each of the time points (24 hours, 48 hours, 72 hours, 96hours). FIG. 19B shows a plot of the total cell area on the surface ofthe nanostructured nitinol coupons compared to the control coupons foreach of the time points (24 hours, 48 hours, 72 hours, 96 hours). FIG.19C shows a plot of the nuclei count per substrate for thenanostructured nitinol coupons compared to the control coupons for eachof the time points (24 hours, 48 hours, 72 hours, 96 hours). FIG. 19Dshows a plot of the total cell area per substrate for the nanostructurednitinol coupons compared to the control coupons for each of the timepoints (24 hours, 48 hours, 72 hours, 96 hours).

FIG. 20 shows a representative current profile resulting from performingthe method described in Example 7.

FIG. 21 shows a representative SEM image of a metal oxide nanostructuredsurface resulting from the method described in Example 7.

FIG. 22 shows a representative current profile resulting from performingthe method described in Example 8.

FIG. 23 shows a representative SEM image of a metal oxide nanostructuredsurface resulting from the method described in Example 8.

DETAILED DESCRIPTION

Illustrative embodiments are now described. Other embodiments may beused in addition or instead. Details that may be apparent or unnecessarymay be omitted to save space or for a more effective presentation. Someembodiments may be practiced with additional components or steps and/orwithout all of the components or steps that are described.

FIGS. 1-5 illustrate one embodiment of the present metal oxidenanostructure 100. The nanostructure 100 can be formed on substantially(e.g. at least about 90%) all of the entire outer surface of a substrate110, or it can be formed on a single or multiple discrete locations onthe surface of a substrate 110.

The nanostructure 100 includes a nanotube layer 120 situated between asubstrate 110 at the nanotube layer's proximal surface 122 and a latticelayer 140 at the nanotube layer's distal surface 124. The nanotube layerincludes a plurality of tubular structures 126 arranged generally orapproximately parallel to one another, running from the proximal surfaceof the nanotube layer to the distal surface of the nanotube layer. Whilethe proximal 122 and distal 124 surfaces are shown as planar or smooth,in practice the surfaces 122 and 124 may or may not be somewhatdiscontinuous. Also, the tubular structures 126 may or may not vary inlength and orientation.

Each of the plurality of tubular structures 126 has a cylindrical wall127 with an inner surface 128, a top surface 129 and an outer surface130, the inner surface 128 defining lumen 132 therein. Collectively, thetop surface 129 of each of the plurality of tubular structures 126defines the distal surface 124 of the nanotube layer 120. The innersurface 128 defines an inner diameter d_(i), and the outer surface 130defines an outer diameter d₀.

Illustrated tubular structures 126 are depicted as right circularcylinders generally aligned with an axis R and perpendicular to surfaces122 and 124. The right circular cylinders define axes 125 that aredefined as the locus of centers of circles defined by the tubularstructures. An axis 125 of a right circular cylinder is therefore alsoperpendicular to surfaces 122 and 124, and the axes 125 are parallel.

The tubular structures 126 are depicted for illustrative purposes asright circular cylinders. However, it is to be understood that tubularstructures 126 only approximate right circular cylinders to a variabledegree. More generally, the cross sections of tubular structures 126 canbe circular, elliptical, polygonal, irregular, or even a variablecombination thereof. A defined axis 125 generally follows thecross-sectional center of a tubular structure 126. The axes of thetubular structures are generally or approximately aligned or defineacute angles with respect to one another. The axes 125 are generallyperpendicular to surfaces 122 and 124 at least to within an acute angle.Also, the cross-sectional geometry of a tubular structure 126 can varyalong its axis 125. It is to be understood that in the followingillustrative description terms like diameters and axes areapproximations to the actual geometry which may embody some degree ofvariation.

In some embodiments, the tubular structures 126 are immediately adjacentto one another, such that there is no observable space between the outersurface 130 of a cylindrical wall 127 of one tubular structure 126 andthe outer surface(s) 130 of the cylindrical wall(s) 127 of the adjacenttubular structure(s) 126. In some embodiments, the outer surface 130 ofthe cylindrical wall 127 of one tubular structure 126 is in directcontact with the outer surface(s) 130 of the cylindrical wall(s) 127 ofthe adjacent tubular structure(s) 126. In some embodiments, the outersurface 130 of the cylindrical wall 127 of one tubular structure 126 isintegrally connected with the outer surface(s) 126 of the cylindricalwall(s) 127 of the adjacent tubular structure(s) 126. In otherembodiments, the tubular structures 126 can be arranged such that thereis observable space between the outer surface 130 of the cylindricalwall 127 of one tubular structure 126 and the outer surface 130 of thecylindrical wall 127 of one or more the adjacent tubular structure(s)126.

In some embodiments, the inner diameter d_(i) is approximately constantalong the height of a given tubular structure 126. In other embodiments,the inner diameter d_(i) changes along the height of a given tubularstructure 126, for example the inner diameter d_(i) is smaller at thetop of the tubular structure 126 than at the bottom of the tubularstructure 126. In some embodiments, each of the plurality of tubularstructures 126 have generally the same inner diameter. In otherembodiments, the tubular structures 126 have varying inner diameters. Insome embodiments, each of the plurality of tubular structures havegenerally the same outer diameter. In other embodiments, the tubularstructures 126 have varying outer diameters. In some embodiments, theinner diameters of the tubular structures 126 are in the range of 10-60nm. In some embodiments, the outer diameters of the tubular structures126 are in the range of 20-80 nm.

The tubular structures 126 also have a height. In some embodiments, theheight is between about 10 nm and 2000 nm. In some embodiments, theheight of the tubular structures 126 is about 400 nm, or between about300 nm to about 500 nm. In other embodiments, the height of the tubularstructures may be between about 100 nm and about 500 nm, about 500 nmand about 1000 nm, or about 1000 nm and about 2000 nm. In otherembodiments, the height of the tubular structures may be about 50 nm,100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 450 nm, 500 nm, 550 nm,600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, or 1000nm.

The tubular structures 126 contain one or more metal oxides. The tubularstructures 126 may or may not further contain carbon, fluoride, or otherelements. In some embodiments, the metal oxide is titanium oxide. Inother embodiments, the metal oxide is nickel oxide. In some embodiments,the metal oxide includes both titanium oxide and nickel oxide. In someembodiments, the ratio of titanium oxide to nickel oxide in the nanotubelayer 120 is greater than or about 1:1.

In some embodiments, the ratio of titanium oxide to nickel oxide in thenanotube layer 120 is greater than 1:1. In other embodiments, the ratioof titanium oxide to nickel oxide in the nanotube layer 120 is less than1:1. In certain embodiments, the ratio of titanium oxide to nickel oxidein the nanotube layer 120 is about 1:1. In some embodiments, the ratioof titanium oxide to nickel oxide in the nanotube layer 120 is greaterthan about 5:1, or between about 5:1 to about 20:1. In some embodiments,the ratio of titanium oxide to nickel oxide in the nanotube layer 120 isgreater than 20:1. In some embodiments, the ratio of titanium oxide tonickel oxide in the nanotube layer 120 is between about 1:1 and 5:1,about 5:1 and 10:1, about 10:1 and 15:1, or about 15:1 and 20:1.

The lattice layer 140 includes a lattice 142 with openings 144 therein.The lattice layer 140 has a proximal surface 146 in contact with orintegrally connected to the distal surface 124 of the nanotube layer120, and a distal surface 148 opposite the proximal surface 146.

In some embodiments, the lattice 142 contains titanium oxide. In otherembodiments, the lattice 142 contains nickel oxide. In some embodiments,the lattice 142 contains both titanium oxide and nickel oxide. In someembodiments, the ratio of titanium oxide to nickel oxide on the distalsurface 148 of the lattice 142 is greater than 5:1, or about 6:1, 7:1,8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1 or20:1. In some embodiments, the ratio of titanium oxide to nickel oxideon the distal surface 148 of the lattice 142 (e.g. the top about 10 nmof the lattice) is about 17:1. In some embodiments, the lattice 142 mayor may not further contain carbon, fluoride, or other elements.

In some embodiments, the ratio of titanium oxide to nickel oxide on thedistal surface 148 the lattice 142 is greater than 1:1. In otherembodiments, the ratio of titanium oxide to nickel oxide on the distalsurface 148 of the lattice 142 is less than 1:1. In certain embodiments,the ratio of titanium oxide to nickel oxide on the distal surface 148 ofthe lattice 142 is about 1:1. In some embodiments, the ratio of titaniumoxide to nickel oxide on the distal surface 148 of the lattice 142 isbetween about 1:1 and 5:1, about 5:1 and 10:1, about 10:1 and 15:1, orabout 15:1 and 20:1.

The lattice layer 140 also has a thickness defined as the averagedistance between the proximal surface 146 and the distal surface 148 ofthe lattice layer 140. In some embodiments, the thickness is betweenabout 5 nm and 100 nm. In some embodiments, the lattice layer 140 has athickness of about 50 nm. In other embodiments, the thickness of thelattice layer 140 may be about 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm,35 nm, 40 nm, 45 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90nm, 95 nm, or 100 nm.

The openings 144 are generally polygonal or irregular (as shown in FIGS.6 and 7 as elements 244 and 344) in shape viewed from above the distalsurface 148 of the lattice layer 140, although in certain embodiments atleast some of the openings 144 may appear to be elliptical or circularwhen viewed from above the distal surface 148 of the lattice layer 140.The openings 144 can be irregularly or variably sized, shaped, andspaced as viewed from the distal surface 148 of the lattice layer 140.However, in certain embodiments, the openings 144 can be nearlyidentical in size, shape, and spacing. In certain embodiments, theaverage space as measured from the center of one openings 144 a to thecenter of an adjacent openings 144 b is approximately 40 nm.

The openings 144 have a hydraulic diameter or a lateral dimension whenthey are not circular. A hydraulic diameter is defined as a ratio offour times an area divided by a perimeter for a given opening. Indiscussing diameters before and hereafter these can be actual diameter,hydraulic diameters, or some interpolation between a major and minoraxis of the opening. In some embodiments, the openings 144 have varyingdiameters. In other embodiments, the openings 144 have generally thesame size diameters. In some embodiments, a majority of the diameters ofthe openings 144 are in the range of about 10 nm to about 60 nm, orabout 20 nm to about 40 nm. In some embodiments a majority of thediameters of the openings 144 can be less than about 10 nm or greaterthan about 60 nm. In some embodiments, the average diameter of theopenings 144 is about 30 nm.

In some embodiments, the centers of the openings 144 are not alignedwith the axes 125 of the tubular structures 126 below. Rather, thenanotube layer 120 and lattice layer 140 can be offset such that thecenters of some of the openings 144 will align with the axes 125 below(e.g. the openings 144 and lumens 132 are generally concentric when viewfrom above the distal surface 148), while the centers of other openings144 will not align with the axes 125 below (e.g. the openings 144 andlumens 132 are generally nonconcentric). In some embodiments, dependingon the size, shape, and spacing of the openings 144, a majority of theopenings 144 can be generally aligned (e.g. the openings 144 and lumens132 are generally concentric when viewed from above the distal surface148) with the lumens 132 of the tubular structures 126 of the nanotubelayer 120 below the lattice layer 140. In other embodiments, a majorityof the openings 144 can be generally unaligned with the lumens 132 (e.g.the openings 144 and lumens 132 are generally nonconcentric). Statedanother way, the axes of the tubular structures 126 may not be centeredupon the openings 144.

The metal oxide nanostructure 100 can be formed on the surface 112 of asubstrate 110 via an electrochemical anodization process. In someembodiments, the substrate 110 can be the outer surface of animplantable medical device, such as a stent. The substrate 110 can alsobe an orthopedic implant, such as a knee implant, bone screw, or bonestaple, such as those used for hand and foot bone fragments osteotomyfixation and joint arthrodesis. In some embodiments, substrate 110 canbe a metal such as titanium, or a titanium alloy including but notlimited to a nickel titanium alloy. In certain embodiments, thesubstrate 110 is made of a nickel titanium alloy in which the ratio ofnickel to titanium near the surface is approximately 1:1. In certainembodiments, the substrate 110 can be a layer of titanium or a titaniumalloy, including but not limited to a nickel titanium alloy, on theouter surface of an implantable medical device. In some embodiments, theouter surface of a substrate 110 may be an oxide layer (e.g. titaniumoxide). The nanostructure 100 can be formed on the entire outer surfaceof the substrate 110, or the nanostructure 100 can be formed on onlycertain portions of the surface of the substrate 110.

FIG. 6 illustrates an alternate embodiment of the lattice layer shown inFIGS. 1-3 with respect to the general shape and arrangement of theopenings 244. The lattice layer 240 contains metal oxides. The latticelayer 240 may or may not further contain carbon, fluoride, or otherelements. The metal oxide material of the lattice layer 240 defines theopenings 244 and forms a lattice 242. In some embodiments, the lattice242 contains titanium oxide. In other embodiments, the lattice 242contains nickel oxide. In some embodiments, the lattice 242 containsboth titanium oxide and nickel oxide. In some embodiments, the ratio oftitanium oxide to nickel oxide on the distal surface 248 of the lattice242 is greater than 5:1, or about 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1,13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1 or 20:1. In some embodiments,the ratio of titanium to nickel oxide on the distal surface 248 of thelattice 242 (e.g. the top about 10 nm of the lattice) is about 17:1.

The openings 244 are generally polygonal in shape viewed from above thedistal surface 248 of the lattice layer 240. The openings 244 can beirregularly sized, shaped, and spaced as viewed from the distal surface248 of the lattice layer 240. However, in certain embodiments, theopenings 244 can be nearly identical in size, shape, and spacing. Incertain embodiments, the average space as measured from the center ofone openings 244 a to the center of an adjacent openings 244 b isapproximately 40 nm.

FIG. 7 illustrates an alternate embodiment of the lattice layer shown inFIGS. 1-3 with respect to the general shape and arrangement of theopenings 344. The lattice layer 340 contains metal oxides. The latticelayer 340 may or may not further contain carbon, fluoride, or otherelements. The metal oxide material of the lattice layer 340 defines theopenings 344 and forms a lattice 342. In some embodiments, the lattice342 contains titanium oxide. In other embodiments, the lattice 342contains nickel oxide. In some embodiments, the lattice 342 containsboth titanium oxide and nickel oxide. In some embodiments, the ratio oftitanium oxide to nickel oxide on the distal surface 348 of the lattice342 is greater than 5:1, or about 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1,13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1 or 20:1. In some embodiments,the ratio of titanium to nickel oxide on the distal surface 348 of thelattice 342 (e.g. the top about 10 nm of the lattice) is about 17:1.

The openings 344 are generally polygonal in shape viewed from above thedistal surface 348 of the lattice layer 340. As shown in FIG. 7 , theopenings 344 can be irregularly sized, shaped, and spaced as viewed fromthe distal surface 348 of the lattice layer 340. In certain embodiments,the average space as measured from the center of one openings 344 a tothe center of an adjacent openings 344 b is approximately 40 nm.

FIG. 8 shows a process flow diagram for one embodiment of a method offorming metal oxide nanostructures. At block 410, an anode and at leastone cathode are optionally provided. In some embodiments, the anodecontains an alloy of nickel and titanium. In some embodiments, the anodecontains an alloy of nickel and titanium, with a ratio of nickel totitanium of approximately 1:1. In some embodiments, the anode can be animplantable medical device, including but not limited to a stent. Theanode can also be an orthopedic implant, such as a knee implant, bonescrew, or bone staple, such as those used for hand and foot bonefragments osteotomy fixation and joint arthrodesis. The at least onecathode(s) can be made from any suitable material, including but notlimited to platinum, iron, stainless steel, or graphite. If more thanone cathode is used, they can be positioned such that they are a similardistance from the anode, and optionally in a symmetrical fashion, andthe setup can optionally include a reference electrode.

At block 420, the anode and cathode(s) are placed in electrical contactthrough a first electrolyte solution. The first electrolyte includes anorganic solvent, a fluoride-bearing species, and an oxygen source. Theoxygen source can be water, or it can be any other single oxygen donorcompound, such as methanol. In some embodiments, the first electrolytemay or may not also optionally contain other additives, such as buffers,surfactants, biocides, salts, and corrosion inhibitors. The firstelectrolyte may or may not also optionally contain an acid, so long asthe acid is weak enough or present at a low enough concentration suchthat it does not interfere with the formation of the nanostructure. Insome embodiments, the first electrolyte does not contain an acid.

The organic solvent can be ethylene glycol. Suitable solvents for useherein include organic solvents, but are not limited to, aliphaticalcohols, aromatic alcohols, diols, glycol ethers, poly(glycol)ethers,lactams, formamides, acetamides, long chain alcohols, ethylene glycol,propylene glycol, diethylene glycols, triethylene glycols, glycerol,dipropylene glycols, glycol butyl ethers, polyethylene glycols,polypropylene glycols, amides, ethers, carboxylic acids, esters,organosulfides, organosulfoxides, sulfones, alcohol derivatives,carbitol, butyl carbitol, cellosolve, ether derivatives, amino alcohols,and ketones. Specific examples of organic solvents include, but are notlimited to, a polyhydric alcohol, such as ethylene glycol, diethyleneglycol, triethylene glycol, tetraethylene glycol, and polyethyleneglycols; a polyhydric alcohol ether, such as ethylene glycolmonomethylether, ethylene glycol monoethylether, ethylene glycolmonobutylether, diethylene glycol monoethylether, diethylene glycolmonobutyl ether, and ethylene glycol monophenyl ether; anitrogen-containing solvent, such as N-methyl-2-pyrrolidone, asubstituted pyrrolidone, and mono-, di-, and tri-ethanolamine; ormixtures thereof. The electrolyte may also include nitrogen-containingketones, such as 2-pyrrolidone, hydroxyethyl-2-pyrrolidone,1,3-dimethylimidazolid-2-one, and octyl-pyrrolidone; diols, such asethanediols, propanediols including 1,2-propanediol, 1,3-propanediol,2-ethyl-2-hydroxymethyl-1,3-propanediol, and ethylhydroxypropanediol,butanediols including 1,2-butanediol, 1,3-butanediol, and1,4-butanediol, pentanediols including 1,2-pentanediol, and1,5-pentanediol, hexanediols including 1,2-hexanediol, 1,6-hexanediol,and 2,5-hexanediol, heptanediols including 1,2-heptanediol, and1,7-heptanediol, octanediols including 1,2-octanediol and1,8-octanediol; alcohols, such as C1-C6 alcohols including methanol,ethanol, propanol, butanol, pentanol, and hexanol, including isomersthereof such as 1-propanol and 2-propanol; glycol ethers and thioglycolethers such as polyalkylene glycols including, but not limited to,propylene glycols such as dipropylene glycol, tripropylene glycol, andtetrapropylene glycol; polymeric glycols such as PEG 200, PEG 300, andPEG 400; thiodiglycol; and mixtures thereof. Additional solvents thatmay be used include hydantoins and derivatives thereof, dimethylsulfoxide, dimethyl sulfone, tetramethylene sulfone, butanetriols suchas 1,2,4-butanetriol, acetic acid, and polyalkoxylated triols. Ionicliquids, such as 1-n-butyl-3-methyl-imidazolium tetrafluoroborate mayalso be used, but they are not preferred due to their high cost.

Suitable fluoride-bearing species include ammonium fluoride, ammoniumbifluoride, potassium fluoride, sodium fluoride, calcium fluoride,magnesium fluoride, and alkylated ammonium fluorides such astetrabutylammonium fluoride, among others.

Suitable solvents and fluoride-bearing species for use herein are thesame as those described for the first anodization step. Suitable acidsfor use herein include mineral acids and organic acids. Examples ofmineral acids include, but are not limited to, HF, HCl, HBr, HI, H₃PO₄,HNO₃, and H₂SO₄. Examples of organic acids include carboxylic acids,including formic acid, adipic acid, fumaric acid, tartaric acid, citricacid, oxalic acid, lactic acid, acetic acid, trifluoroacetic acid, andothers.

The first electrolyte solution can be maintained at a relativelyconstant temperature. The temperature of the first electrolyte solutioncan be between about 10° and 50° Celsius. In some embodiments, thetemperature can be between about 20° and 40° Celsius.

In some embodiments, the first electrolyte solution includes about 99.2vol % organic solvent and about 0.8 vol % water, about 0.18 wt %fluoride-bearing species, and is maintained at about 30° C. In certainembodiments, the first electrolyte solution includes 99.2 ml organicsolvent, 0.2 g fluoride salt, and 0.8 ml water, and is maintained atabout 30° C.

At block 430, a first voltage V₁ is applied across the anode and cathodethrough the first electrolyte solution for a first time period t₁. Thefirst voltage V₁ can be between about 10V and about 60V. In someembodiments, the first voltage V₁ is about 25V. In some embodiments, thefirst voltage applied across the anode and cathode is constant for thefirst time period t₁. In other embodiments, the first voltage appliedacross the anode and cathode varies over time throughout the first timeperiod t₁, as for example when the anodization is run in a galvanostaticmode. The first time period t₁ can be between about 1 minute and about120 minutes. In some embodiments, the first time period t₁ isapproximately 5 minutes. In some embodiments, the first time period t₁is less than 5 minutes. In other embodiments, the first time period isgreater than 5 minutes, including about 6, 7, 8, 9, or 10 minutes. Insome embodiments, the first time period is greater than 10 minutes andless than 60 minutes. In some embodiments, the first time period isgreater than 60 minutes.

At block 440, the anode and cathode(s) are placed in electrical contactthrough a second electrolyte solution. The second electrolyte solutionincludes an organic solvent, a fluoride-bearing species, an oxygensource, and an acid. The organic solvent can be ethylene glycol. In someembodiments, the fluoride salt can be ammonium fluoride. The oxygensource can be water, or it can be any other single oxygen donorcompound, such as methanol. The acid can be selected from the groupconsisting of H₂SO₄, HF, HCl, HBr, HI, H₃PO₄, HNO₃, formic acid, adipicacid, fumaric acid, tartaric acid, citric acid, oxalic acid, lacticacid, acetic acid, trifluoroacetic acid, and others. In someembodiments, the acid is sulfuric acid (H₂SO₄). In some embodiments, theacid is hydroiodic acid (HI). The second electrolyte solution includesgreater than about 90 vol % organic solvent, about 0.8 vol % water,about 0.001-9.0 vol % acid, and about 0.18 wt % fluoride-bearingspecies. The appropriate percentage of acid in the second electrolytesolution depends on many factors including the acid(s) used and themolecular weight and pKa of said acid(s). The second electrolytesolution can be maintained at a relatively constant temperature. Thetemperature of the second electrolyte solution can be between about 10°and 50° Celsius. In some embodiments, the temperature of the secondelectrolyte solution is about 30° C. In some embodiments, the firstelectrolyte solution may be modified by lowering the pH or increasingthe acid concentration to achieve the second electrolyte solution.

At block 450, a second voltage V₂ is applied across the anode andcathode through the second electrolyte solution for a second timeperiod. In some embodiments, the second voltage is about 25V. In someembodiments, the second voltage V₂ is constant throughout the secondtime period t₂. In other embodiments, the second voltage V₂ variesthroughout the second time period t₂. The second time period t₂ can bebetween about 1 minute and about 120 minutes. In some embodiments, thesecond time period t₂ is approximately 5 minutes. In some embodiments,the second time period t₂ is less than 5 minutes. In other embodiments,the second time period t₂ is greater than 5 minutes, including about 6,7, 8, 9, or 10 minutes. In some embodiments, the second time period t₂is greater than 10 minutes and less than 60 minutes. In someembodiments, the second time period t₂ is greater than 60 minutes.

Under suboptimal conditions (e.g. suboptimal acidity in secondelectrolyte solution, suboptimal voltage V₁ or V₂, suboptimaltemperature, or suboptimal first or second time period t₁ or t₂),pitting corrosion and/or amorphous material or particulate may or maynot be observed. In some embodiments, pitting corrosion and/or amorphousmaterial or particulate are substantially absent from the resultingsurface.

FIG. 9 shows a process flow diagram for another embodiment of a methodof forming metal oxide nanostructures. At block 510, an anode and atleast one cathode are optionally provided. In some embodiments, theanode contains an alloy of nickel and titanium. In some embodiments, theanode contains an alloy of nickel and titanium, with a ratio of nickelto titanium of approximately 1:1. In some embodiments, the anode can bean implantable medical device, including but not limited to a stent. Theanode can also be an orthopedic implant, such as a knee implant, bonescrew, or bone staple, such as those used for hand and foot bonefragments osteotomy fixation and joint arthrodesis. The at least onecathode(s) can be made from any suitable material, including but notlimited to platinum, iron, stainless steel, or graphite. If more thanone cathode is used, they can be positioned such that they are a similardistance from the anode, and optionally in a symmetrical fashion, andthe setup can optionally include a reference electrode.

At block 520, the anode and cathode(s) are placed in electrical contactthrough a first electrolyte solution. The first electrolyte solutionincludes an organic solvent, a fluoride-bearing species, and an oxygensource. The organic solvent can be ethylene glycol. In some embodiments,the fluoride-bearing species can be ammonium fluoride. The oxygen sourcecan be water, or it can be any other single oxygen donor compound, suchas methanol. In some embodiments, the first electrolyte does not includean acid. In some embodiments, the first electrolyte solution includesabout 99.2 vol % organic solvent and about 0.8 vol % water, and about0.18 wt % fluoride-bearing species. The first electrolyte solution canbe maintained at a relatively constant temperature. The temperature ofthe first electrolyte solution can be between about 10° and 50° Celsius.In some embodiments, the temperature of the first electrolyte solutionis about 30° C.

At block 530, a first voltage V₁ is applied across the anode and cathodethrough the first electrolyte solution for a first time period t₁. Thefirst voltage V₁ can be between about 10V and about 60V. In someembodiments, the first voltage V₁ is about 25V. In some embodiments, thefirst voltage applied across the anode and cathode is constant for thefirst time period t₁. In other embodiments, the first voltage appliedacross the anode and cathode varies over time throughout the first timeperiod t₁, as for example when the anodization is run in a galvanostaticmode.

The first time period t₁ can be between about 1 minute and about 120minutes. In some embodiments, the first time period t₁ is approximately5 minutes. In some embodiments, the first time period t₁ is less than 5minutes. In other embodiments, the first time period is greater than 5minutes, including about 6, 7, 8, 9, or 10 minutes. In some embodiments,the first time period is greater than 10 minutes and less than 60minutes. In some embodiments, the first time period is greater than 60minutes.

At block 540, the first electrolyte solution is modified by adding anacid resulting in a second electrolyte solution. The acid can beselected from the group consisting of H₂SO₄, HF, HCl, HBr, HI, H₃PO₄,HNO₃, formic acid, adipic acid, fumaric acid, tartaric acid, citricacid, oxalic acid, lactic acid, acetic acid, trifluoroacetic acid, andothers. In some embodiments, the acid is sulfuric acid (H₂SO₄). In someembodiments, the acid is hydroiodic acid (HI). The second electrolytesolution includes greater than about 90 vol % organic solvent, about 0.8vol % water, about 0.001-9.0 vol % acid, and fluoride-bearing species(wt % dependent on acid used in second electrolyte solution). Theappropriate percentage of acid in the second electrolyte solutiondepends on many factors including the acid(s) used and the molecularweight and pKa of said acid(s). The second electrolyte solution can bemaintained at a relatively constant temperature. The temperature of thesecond electrolyte solution can be between about 10° and 50° Celsius. Insome embodiments, the temperature of the second electrolyte solution isabout 30° C. In an alternate embodiment, the first electrolyte solutionmay be modified by lowering the pH or increasing the acid concentration.

At block 550, a second voltage V₂ is applied across the anode andcathode through the second electrolyte solution for a second time periodt₂. The second voltage V₂ can be between about 10V and about 60V. Insome embodiments, the second voltage V₂ is about 25V. In someembodiments, the second voltage V₂ is constant throughout the secondtime period t₂. In other embodiments, the second voltage V₂ variesthroughout the second time period t₂. The second time period t₂ can bebetween about 1 minute and about 120 minutes. In some embodiments, thesecond time period t₂ is approximately 5 minutes. In some embodiments,the second time period t₂ is less than 5 minutes. In other embodiments,the second time period t₂ is greater than 5 minutes, including about 6,7, 8, 9, or 10 minutes. In some embodiments, the second time period t₂is greater than 10 minutes and less than 60 minutes. In someembodiments, the second time period t₂ is greater than 60 minutes.

Under suboptimal conditions (e.g. suboptimal acidity in secondelectrolyte solution, suboptimal voltage V₁ or V₂, suboptimaltemperature, or suboptimal first or second time period t₁ or t₂),pitting corrosion and/or amorphous material or particulate may or maynot be observed. In some embodiments, pitting corrosion and/or amorphousmaterial or particulate are substantially absent from the resultingsurface.

FIG. 10 shows a process flow diagram for another embodiment of a methodof forming metal oxide nanostructures. At block 610, an anode and atleast one cathode are optionally provided. In some embodiments, theanode contains an alloy of nickel and titanium. In some embodiments, theanode contains an alloy of nickel and titanium, with a ratio of nickelto titanium of approximately 1:1. In some embodiments, the anode can bean implantable medical device, including but not limited to a stent. Theanode can also be an orthopedic implant, such as a knee implant, bonescrew, or bone staple, such as those used for hand and foot bonefragments osteotomy fixation and joint arthrodesis. The at least onecathode(s) can be made from any suitable material, including but notlimited to platinum, iron, stainless steel, or graphite. If more thanone cathode is used, they can be positioned such that they are a similardistance from the anode, and optionally in a symmetrical fashion, andthe setup can optionally include a reference electrode.

At block 620, the anode and cathode(s) are placed in electrical contactthrough a first electrolyte solution. The first electrolyte solutionincludes an organic solvent, a fluoride-bearing species, and an oxygensource. The organic solvent can be ethylene glycol. In some embodiments,the fluoride-bearing species can be ammonium fluoride. The oxygen sourcecan be water, or it can be any other single oxygen donor compound, suchas methanol. In some embodiments, the first electrolyte does not includean acid. In some embodiments, the first electrolyte solution includesabout 99.2 vol % organic solvent and 0.8 vol % water, and about 0.18 wt% fluoride-bearing species. The first electrolyte solution can bemaintained at a relatively constant temperature. The temperature of thefirst electrolyte solution can be between about 10° and 50° Celsius. Insome embodiments, the temperature of the first electrolyte solution isabout 30° C.

At block 630, a first voltage V₁ is applied across the anode and cathodethrough the first electrolyte solution for a first time period t₁. Thefirst voltage V₁ can be between about 10V and about 60V. In someembodiments, the first voltage V₁ is about 25V. In some embodiments, thefirst voltage applied across the anode and cathode is constant for thefirst time period t₁. In other embodiments, the first voltage appliedacross the anode and cathode varies over time throughout the first timeperiod t₁, as for example when the anodization is run in a galvanostaticmode.

The first time period t₁ can be between about 1 minute and about 120minutes. In some embodiments, the first time period t₁ is approximately5 minutes. In some embodiments, the first time period t₁ is less than 5minutes. In other embodiments, the first time period is greater than 5minutes, including about 6, 7, 8, 9, or 10 minutes. In some embodiments,the first time period is greater than 10 minutes and less than 60minutes. In some embodiments, the first time period is greater than 60minutes.

At block 640, the first electrolyte solution is removed and replaced asecond electrolyte solution including an organic solvent, afluoride-bearing species, an oxygen source, and an acid. The organicsolvent can be ethylene glycol. In some embodiments, thefluoride-bearing species can be ammonium fluoride. The oxygen source canbe water, or it can be any other single oxygen donor compound, such asmethanol. The acid can be selected from the group consisting of H₂SO₄,HF, HCl, HBr, HI, H₃PO₄, HNO₃, formic acid, adipic acid, fumaric acid,tartaric acid, citric acid, oxalic acid, lactic acid, acetic acid,trifluoroacetic acid, and others. In some embodiments, the acid issulfuric acid (H₂SO₄). In some embodiments, the acid is hydroiodic acid(HI). The second electrolyte solution includes greater than about 90 vol% organic solvent, about 0.8 vol % water, about 0.001-9.0 vol % acid,and fluoride-bearing species (wt % dependent on acid used in secondelectrolyte solution). The appropriate percentage of acid in the secondelectrolyte solution depends on many factors including the acid(s) usedand the molecular weight and pKa of said acid(s). The second electrolytesolution can be maintained at a relatively constant temperature. Thetemperature of the second electrolyte solution can be between about 10°and 50° Celsius. In some embodiments, the temperature of the secondelectrolyte solution is about 30° C.

At block 650, a second voltage V₂ is applied across the anode andcathode through the second electrolyte solution for a second time periodt₂. The second voltage V₂ can be between about 10V and about 60V. Insome embodiments, the second voltage V₂ is about 25V. In someembodiments, the second voltage V₂ is constant throughout the secondtime period t₂. In other embodiments, the second voltage V₂ variesthroughout the second time period t₂. The second time period t₂ can bebetween about 1 minute and about 120 minutes. In some embodiments, thesecond time period t₂ is approximately 5 minutes. In some embodiments,the second time period t₂ is less than 5 minutes. In other embodiments,the second time period t₂ is greater than 5 minutes, including about 6,7, 8, 9, or 10 minutes. In some embodiments, the second time period t₂is greater than 10 minutes and less than 60 minutes. In someembodiments, the second time period t₂ is greater than 60 minutes.

Under suboptimal conditions (e.g. suboptimal acidity in secondelectrolyte solution, suboptimal voltage V₁ or V₂, suboptimaltemperature, or suboptimal first or second time period t₁ or t₂),pitting corrosion and/or amorphous material or particulate may or maynot be observed. In some embodiments, pitting corrosion and/or amorphousmaterial or particulate are substantially absent from the resultingsurface.

FIG. 11 shows a process flow diagram for one embodiment of a method offorming metal oxide nanostructures. At block 710, a nickel titaniumanode and at least one cathode are optionally provided. The anode can bean implantable medical device, including but not limited to a stent,containing a nickel titanium alloy. The anode can also be an orthopedicimplant, such as a knee implant, bone screw, or bone staple, such asthose used for hand and foot bone fragments osteotomy fixation and jointarthrodesis. The cathode can be platinum.

At block 720, the anode and cathode(s) are placed in electrical contactthrough a first electrolyte solution including about 99.2 vol % ethyleneglycol (e.g. 99.2 ml) and about 0.8 vol % water (e.g. 0.8 ml), and about0.18 wt % ammonium fluoride (e.g. 0.2 g). The temperature of the firstelectrolyte solution is maintained at about 30° C.

At block 730, a first voltage of about 25V is applied across the anodeand cathode(s) through the first electrolyte solution for about 5minutes.

At block 740, the first electrolyte solution is modified by the additionof hydroiodic acid, resulting in a second electrolyte solution includingethylene glycol (e.g. about 98.8 wt %), fluoride-bearing species (e.g.about 0.20 wt %), water (e.g. about 0.8 wt %), and hydroiodic acid (e.g.0.06 mmol (8 ul) 57 wt % hydroiodic acid per 100 ml of firstelectrolyte). The organic solvent can be ethylene glycol. Thefluoride-bearing species can be ammonium fluoride. The temperature ofthe second electrolyte solution can be maintained at about 30° C.

At block 750, a second voltage of about 25V is applied across the anodeand cathode(s) through the second electrolyte solution for about 5minutes.

Under suboptimal conditions (e.g. suboptimal acidity in secondelectrolyte solution, suboptimal voltage V₁ or V₂, suboptimaltemperature, or suboptimal first or second time period t₁ or t₂),pitting corrosion and/or amorphous material or particulate may or maynot be observed. In some embodiments, pitting corrosion and/or amorphousmaterial or particulate are substantially absent from the resultingsurface.

FIG. 12 shows a process flow diagram for one embodiment of a method offorming metal oxide nanostructures. At block 810, an anode and at leastone cathode are optionally provided. In some embodiments, the anodecontains an alloy of nickel and titanium. In some embodiments, the anodecontains an alloy of nickel and titanium, with a ratio of nickel totitanium of approximately 1:1. In some embodiments, the anode can be animplantable medical device, including but not limited to a stent. Theanode can also be an orthopedic implant, such as a knee implant, bonescrew, or bone staple, such as those used for hand and foot bonefragments osteotomy fixation and joint arthrodesis. The at least onecathode(s) can be made from any suitable material, including but notlimited to platinum, iron, stainless steel, or graphite. If more thanone cathode is used, they can be positioned such that they are a similardistance from the anode, and optionally in a symmetrical fashion, andthe setup can optionally include a reference electrode.

At block 820, the anode and cathode(s) are placed in electrical contactthrough a first electrolyte solution. The first electrolyte solutionincludes an organic solvent, a fluoride-bearing species, and an oxygensource. The organic solvent can be ethylene glycol. In some embodiments,the fluoride-bearing species can be ammonium fluoride. The oxygen sourcecan be water, or it can be any other single oxygen donor compound, suchas methanol. In some embodiments, the first electrolyte does not includean acid. In some embodiments, the first electrolyte solution includesabout 99.2 vol % organic solvent and about 0.8 vol % water, and about0.18 wt % fluoride-bearing species. The first electrolyte solution canbe maintained at a relatively constant temperature. The temperature ofthe first electrolyte solution can be between about 10° and 50° Celsius.In some embodiments, the temperature of the first electrolyte solutionis about 30° C.

At block 830, a first voltage V₁ is applied across the anode and cathodethrough the first electrolyte solution for a first time period t₁. Thefirst voltage V₁ can be between about 10V and about 60V. In someembodiments, the first voltage V₁ is about 25V. In some embodiments, thefirst voltage applied across the anode and cathode is constant for thefirst time period t₁. In other embodiments, the first voltage appliedacross the anode and cathode varies over time throughout the first timeperiod t₁, as for example when the anodization is run in a galvanostaticmode. The first time period t₁ can be between about 1 minute and about120 minutes. In some embodiments, the first time period t₁ isapproximately 5 minutes. In some embodiments, the first time period t₁is less than 5 minutes. In other embodiments, the first time period isgreater than 5 minutes, including about 6, 7, 8, 9, or 10 minutes. Insome embodiments, the first time period is greater than 10 minutes andless than 60 minutes. In some embodiments, the first time period isgreater than 60 minutes.

At block 840, at least a portion of the anodized surface (e.g. oxidematerial) of the anode is removed or modified. A portion of the anodizedsurface (e.g. oxide material) can be removed or modified by a variety ofacceptable methods, including but not limited to etching,electrochemical anodization, and ultrasound. In other embodiments, aportion of the anodized surface (e.g. oxide material) can be removed ormodified by cleaning it (e.g. ultrasonic cleaning, plasma cleaning),rinsing it (e.g. with water or other solvent), or annealing. In someembodiments, the oxide layer (e.g. nanotubes) formed during the firsttime period can be completely or partially removed from the substrate,leaving the resulting surface of the substrate characterized by apattern of nanopores or nanopits.

At block 850, the anode and cathode(s) are placed in electrical contactthrough a second electrolyte solution. The second electrolyte solutionincludes an organic solvent, a fluoride-bearing species, an oxygensource, and an acid. The organic solvent can be ethylene glycol. In someembodiments, the fluoride-bearing species can be ammonium fluoride. Theoxygen source can be water, or it can be any other single oxygen donorcompound, such as methanol. The acid can be selected from the groupconsisting of H₂SO₄, HF, HCl, HBr, HI, H₃PO₄, HNO₃, formic acid, adipicacid, fumaric acid, tartaric acid, citric acid, oxalic acid, lacticacid, acetic acid, trifluoroacetic acid, and others. In someembodiments, the acid is sulfuric acid (H₂SO₄). In some embodiments, theacid is hydroiodic acid (HI). The second electrolyte solution includesgreater than about 90 vol % organic solvent, about 0.8 vol % water,about 0.001-9.0 vol % acid, and fluoride-bearing species (wt % dependenton acid used in second electrolyte solution). The appropriate percentageof acid in the second electrolyte solution depends on many factorsincluding the acid(s) used and the molecular weight and pKa of saidacid(s). The second electrolyte solution can be maintained at arelatively constant temperature. The temperature of the secondelectrolyte solution can be between about 10° and 50° Celsius. In someembodiments, the temperature of the second electrolyte solution is about30° C.

At block 860, a second voltage V₂ is applied across the anode andcathode through the second electrolyte solution for a second timeperiod. In some embodiments, the second voltage is about 25V. In someembodiments, the second voltage V₂ is constant throughout the secondtime period t₂. In other embodiments, the second voltage V₂ variesthroughout the second time period t₂. The second time period t₂ can bebetween about 1 minute and about 120 minutes. In some embodiments, thesecond time period t₂ is approximately 5 minutes. In some embodiments,the second time period t₂ is less than 5 minutes. In other embodiments,the second time period t₂ is greater than 5 minutes, including about 6,7, 8, 9, or 10 minutes. In some embodiments, the second time period t₂is greater than 10 minutes and less than 60 minutes. In someembodiments, the second time period t₂ is greater than 60 minutes.

Under suboptimal conditions (e.g. suboptimal acidity in secondelectrolyte solution, suboptimal voltage V₁ or V₂, suboptimaltemperature, or suboptimal first or second time period t₁ or t₂),pitting corrosion and/or amorphous material or particulate may or maynot be observed. In some embodiments, pitting corrosion and/or amorphousmaterial or particulate are substantially absent from the resultingsurface.

FIG. 13 shows a process flow diagram for one embodiment of a method offorming metal oxide nanostructures. At block 910, an anode and at leastone cathode are optionally provided. In some embodiments, the anodecontains an alloy of nickel and titanium. In some embodiments, the anodecontains an alloy of nickel and titanium, with a ratio of nickel totitanium of approximately 1:1. In some embodiments, the anode can be animplantable medical device, including but not limited to a stent. Theanode can also be an orthopedic implant, such as a knee implant, bonescrew, or bone staple, such as those used for hand and foot bonefragments osteotomy fixation and joint arthrodesis. The at least onecathode(s) can be made from any suitable material, including but notlimited to platinum, iron, stainless steel, or graphite. If more thanone cathode is used, they can be positioned such that they are a similardistance from the anode, and optionally in a symmetrical fashion, andthe setup can optionally include a reference electrode.

At block 920, the anode and cathode(s) are placed in electrical contactthrough an electrolyte solution. The electrolyte solution includes anorganic solvent, a fluoride-bearing species, and an oxygen source. Theorganic solvent can be ethylene glycol. In some embodiments, thefluoride-bearing species can be ammonium fluoride. The oxygen source canbe water, or it can be any other single oxygen donor compound, such asmethanol. In some embodiments, the electrolyte does not include an acid.In some embodiments, the electrolyte solution includes about 99.2 vol %organic solvent and about 0.8 vol % water, and about 0.18 wt %fluoride-bearing species. The electrolyte solution can be maintained ata relatively constant temperature. The temperature of the electrolytesolution can be between about 10° and 50° Celsius. In some embodiments,the temperature of the electrolyte solution is about 30° C.

At block 930, a first voltage V₁ is applied across the anode and cathodethrough the electrolyte solution for a first time period t₁. The firstvoltage V₁ can be between about 10V and about 60V. In some embodiments,the first voltage V₁ is about 25V. In some embodiments, the firstvoltage applied across the anode and cathode is constant for the firsttime period t₁. In other embodiments, the first voltage applied acrossthe anode and cathode varies over time throughout the first time periodt₁, as for example when the anodization is run in a galvanostatic mode.The first time period t₁ can be between about 1 minute and about 120minutes. In some embodiments, the first time period t₁ is approximately5 minutes. In some embodiments, the first time period t₁ is less than 5minutes. In other embodiments, the first time period is greater than 5minutes, including about 6, 7, 8, 9, or 10 minutes. In some embodiments,the first time period is greater than 10 minutes and less than 60minutes. In some embodiments, the first time period is greater than 60minutes.

At block 940, at least a portion of the anodized surface (e.g. oxidematerial) of the anode is removed or modified. A portion of the anodizedsurface (e.g. oxide material) can be removed or modified by a variety ofacceptable methods, including but not limited to etching,electrochemical anodization, and ultrasound. In other embodiments, aportion of the anodized surface (e.g. oxide material) can be removed ormodified by cleaning it (e.g. ultrasonic cleaning, plasma cleaning),rinsing it (e.g. with water or other solvent), or annealing. In someembodiments, the oxide layer (e.g. nanotubes) formed during the firsttime period can be completely or partially removed from the substrate,leaving the resulting surface of the substrate characterized by apattern of nanopores or nanopits.

At block 950, the anode and cathode(s) are again placed in electricalcontact through the electrolyte solution. The electrolyte solutionincludes an organic solvent, a fluoride-bearing species, and an oxygensource. The organic solvent can be ethylene glycol. In some embodiments,the fluoride-bearing species can be ammonium fluoride. The oxygen sourcecan be water, or it can be any other single oxygen donor compound, suchas methanol. In some embodiments, the electrolyte does not include anacid. In some embodiments, the electrolyte solution includes about 99.2vol % organic solvent and about 0.8 vol % water, and about 0.18 wt %fluoride-bearing species. The electrolyte solution can be maintained ata relatively constant temperature. The temperature of the electrolytesolution can be between about 10° and 50° Celsius. In some embodiments,the temperature of the electrolyte solution is about 30° C.

At block 960, a second voltage V₂ is applied across the anode andcathode through the second electrolyte solution for a second timeperiod. In some embodiments, the second voltage is about 25V. In someembodiments, the second voltage V₂ is constant throughout the secondtime period t₂. In other embodiments, the second voltage V₂ variesthroughout the second time period t₂. The second time period t₂ can bebetween about 1 minute and about 120 minutes. In some embodiments, thesecond time period t₂ is approximately 5 minutes. In some embodiments,the second time period t₂ is less than 5 minutes. In other embodiments,the second time period t₂ is greater than 5 minutes, including about 6,7, 8, 9, or 10 minutes. In some embodiments, the second time period t₂is greater than 10 minutes and less than 60 minutes. In someembodiments, the second time period t₂ is greater than 60 minutes.

Under suboptimal conditions (e.g. suboptimal acidity in secondelectrolyte solution, suboptimal voltage V₁ or V₂, suboptimaltemperature, or suboptimal first or second time period t₁ or t₂),pitting corrosion and/or amorphous material or particulate may or maynot be observed. In some embodiments, pitting corrosion and/or amorphousmaterial or particulate are substantially absent from the resultingsurface.

The components, steps, features, objects, benefits, and advantages thathave been discussed are merely illustrative. None of them, nor thediscussions relating to them, are intended to limit the scope ofprotection in any way. Numerous other embodiments are also contemplated.These include embodiments that have fewer, additional, and/or differentcomponents, steps, features, objects, benefits, and/or advantages. Thesealso include embodiments in which the components and/or steps arearranged and/or ordered differently.

Example 1. Forming Metal Oxide Nanostructures on Nickel Titanium Foil

An electrolyte solution was prepared containing 0.8 vol % deionizedwater (H₂O) (e.g. 0.8 ml), 0.18 wt % ammonium fluoride (NH₄F) (e.g. 0.2g) (Sigma Aldrich), and 99.2 vol % ethylene glycol (e.g. 99.2 ml) (SigmaAldrich). The electrolyte solution was brought to and maintained at 30°C.

50 mm×50 mm×0.127 mm (W×H×D) nitinol foil (Alfa Aesar) was cut into 6mm×6 mm×0.127 mm (W×H×D) and 8 mm×8 mm×0.127 mm (W×H×D) coupons and thensuccessively ultrasonically cleaned with acetone, ethanol, and deionizedwater for 5 minutes each. The coupons were then kept in 70% ethanoluntil further use. Prior to anodization, the coupons were rinsed indeionized water and air dried.

For anodization, the cleaned nitinol coupons were then secured such thatonly one face of the coupon was exposed to the reaction conditions. Thesecured nitinol coupons were positioned approximately 2 cm from aplatinum cathode (Sigma Aldrich). The platinum cathode had the samesurface area as the nitinol coupon being anodized (e.g. 6 mm×6 mm or 8mm×8 mm).

A power supply (Agilent Technologies) provided a constant voltage of 25Vbetween the anode (nitinol coupon) and platinum cathode for 5 minutes.During this time, the current was variable as monitored by a digitalmultimeter (Agilent Technologies) connected to a desktop computerrunning BenchVue 3.1 (Keysight Technologies). A representative currentprofile (including the current from the second time period) is shown inFIG. 14 .

After the first 5 minute anodization, 0.06 mmol (8 ul) 57 wt %hydroiodic acid (Sigma Aldrich) was added to the electrolyte solutionfrom the first step. The electrolyte solution continued to be maintainedat 30° C. The power supply then provided a constant voltage of 25Vbetween the anode (nitinol coupon) and cathode for another 5 minutes.During this second 5-minute run, the electrolyte solution was stirredusing magnetic PTFE-coated stir bar at 300 rpm.

After the second 5-minute run, the coupons were rinsed in deionizedwater and kept in 70% ethanol until further use or evaluation. Thecoupons were then imaged using scanning electron microscopy (SEM)(representative image shown in FIG. 15 ).

Example 2. Forming Metal Oxide Nanostructures on Nickel Titanium Foil

An electrolyte solution was prepared containing 0.8 vol % deionizedwater (H₂O) (e.g. 0.8 ml), 0.18 wt % ammonium fluoride (NH₄F) (e.g. 0.2g) (Sigma Aldrich), and 99.2 vol % ethylene glycol (e.g. 99.2 ml) (SigmaAldrich). The electrolyte solution was brought to and maintained at 30°C.

50 mm×50 mm×0.127 mm (W×H×D) nitinol foil (Alfa Aesar) was cut into 6mm×6 mm×0.127 mm (W×H×D) and 8 mm×8 mm×0.127 mm (W×H×D) coupons and thensuccessively ultrasonically cleaned with acetone, ethanol, and deionizedwater for 5 minutes each. The coupons were then kept in 70% ethanoluntil further use. Prior to anodization, the coupons were rinsed indeionized water and air dried.

For anodization, the cleaned nitinol coupons were then secured such thatonly one face of the coupon was exposed to the reaction conditions. Thesecured nitinol coupons were positioned approximately 2 cm from aplatinum cathode (Sigma Aldrich). The platinum cathode had the samesurface area as the nitinol coupon being anodized (e.g. 6 mm×6 mm or 8mm×8 mm).

A power supply (Agilent Technologies) provided a constant voltage of 25Vbetween the anode (nitinol coupon) and platinum cathode for 5 minutes.During this time, the current was variable as monitored by a digitalmultimeter (Agilent Technologies) connected to a desktop computerrunning BenchVue 3.1 (Keysight Technologies).

After the first 5 minute anodization, 3 mmol (163.2 ul) 95-98 wt %sulfuric acid (VWR) was added to the electrolyte solution from the firststep. The electrolyte solution continued to be maintained at 30° C. Thepower supply then provided a constant voltage of 25V between the anode(nitinol coupon) and cathode for another 5 minutes. During this second5-minute run, the electrolyte solution was stirred using magneticPTFE-coated stir bar at 300 rpm.

After the second 5 minute run, the coupons were rinsed in deionizedwater and kept in 70% ethanol until further use or evaluation. Some ofthe coupons were then imaged using SEM (representative image shown inFIG. 16 ).

Example 3. Forming Metal Oxide Nanostructures on Nickel Titanium Foil

An electrolyte solution was prepared containing 0.8 vol % deionizedwater (H₂O) (e.g. 0.8 ml), 0.18 wt % ammonium fluoride (NH₄F) (e.g. 0.2g) (Sigma Aldrich), and 99.2 vol % ethylene glycol (e.g. 99.2 ml) (SigmaAldrich). The electrolyte solution was brought to and maintained at 22°C.

50 mm×50 mm×0.127 mm (W×H×D) nitinol foil (Alfa Aesar) was cut into 6mm×6 mm×0.127 mm (W×H×D) and then successively ultrasonically cleanedwith acetone, ethanol, and deionized water for 5 minutes each. Thecoupons were then kept in 70% ethanol until further use. Prior toanodization, the coupons were rinsed in deionized water and air dried.

For anodization, the cleaned nitinol coupons were then secured such thatonly one face of the coupon was exposed to the reaction conditions. Thesecured nitinol coupons were positioned approximately 2 cm from aplatinum cathode (Sigma Aldrich). The platinum cathode had the samesurface area as the nitinol coupon being anodized (e.g. 6 mm×6 mm).

A power supply (Agilent Technologies) provided a constant voltage of 25Vbetween the anode (nitinol coupon) and platinum cathode for 5 minutes.During this time, the current was variable as monitored by a digitalmultimeter (Agilent Technologies) connected to a desktop computerrunning BenchVue.

After the first 5 minute anodization, 0.06 mmol (8 ul) 57 wt %hydroiodic acid (Sigma Aldrich) was added to the electrolyte solutionfrom the first step. The electrolyte solution continued to be maintainedat 22° C. The power supply then provided a constant voltage of 25Vbetween the anode (nitinol coupon) and cathode for another 7 minutes.During this second 7-minute run, the electrolyte solution was stirredusing magnetic PTFE-coated stir bar at 300 rpm.

After the second 5-minute run, the coupons were rinsed in deionizedwater and kept in 70% ethanol until further use or evaluation.

Example 4. Determining the Elemental Composition of the Surface of theOxide Nanostructures

50 mm×50 mm×0.127 mm (W×H×D) nitinol foil (Alfa Aesar) was cut into 6mm×6 mm×0.127 mm (W×H×D) coupons. One of these 6 mm×6 mm×0.127 mm(W×H×D) nitinol foil coupons (Alfa Aesar) was then prepared as describedin Example 1, and three discrete spots on this coupon were quantifiedusing standard x-ray photoelectron spectroscopy (XPS) to determine therelative nickel, titanium carbon, and oxygen content at the surface ofthe anodized nitinol coupon. The results found by XPS are shown in Table1.

TABLE 1 Relative Atomic Concentration (%) Sample Ni Ti C O 1 2.3 39.5 3028.2 2 2.5 41.1 26.3 30.1 3 2.1 41.3 27.1 29.5

Example 5. Endothelial Response to Oxide Nanostructures

Human aortic endothelial cells (HAECs) (passage 4) were cultured inwells with standard techniques on 500 um thick 4 mg/ml collagensubstrate (rat tail Corning #354236) at a seeding density of 10,000cells per cm² until confluent (approx. 2-3 days).

FIG. 17 is a schematic representation of the well setup 20. Asrepresented in FIG. 17 , an insert 30 (e.g. nanostructured nitinolcoupon, control coupon) was pushed into the collagen substrate 40 withHAECs 50, allowing the HAECs 50 to migrate onto the surface of theinsert 30. The well setup was then imaged along various focal planes 70,80, and 90.

Twenty 6 mm×6 mm×0.127 mm (W×H×D) nitinol foil coupons (Alfa Aesar)prepared as described in Example 3 above were cleaned three times eachwith 1 mL sterile water. The coupons were then inserted into 20 wells(one coupon per well) with confluent HAEC layers, as shown in FIG. 17 ,with the oxide nanostructures facing up (away from the confluent HAEClayer). 20 untreated 6 mm×6 mm×0.127 mm (W×H×D) nitinol coupons(control) were then cleaned three times each with 1 mL sterile water,and then inserted into 20 wells (one coupon per well) with confluentHAEC layers, as shown in FIG. 17 , with a clean native oxide layerfacing up.

At time periods of 24 hours, 48 hours, 72 hours, and 96 hourspost-coupon insertion, wells were stained with CellTracker Green for 30minutes, fixed in 4% PFA, and counter-stained with Hoechst 33342, anuclear stain, for 20 minutes (five nanostructured coupons and 5 controlcoupons per time point). The wells were then imaged with ThermoScientific CX7 automated quantitative fluorescence microscope.Measurements were made to determine cell count, average cell area, andcell morphology (e.g. P2A, LWR). For purposes of these measurements,only cell nuclei associated with CellTracker Green were counted as livecells. Compared to the control coupons, the coupons with the oxidenanostructures were found to have about 3 times more live cells at 96hours (Tables 2 & 3; FIGS. 18A-D).

TABLE 2 Sample Total Nuclei Count Control #1 529.92 Control #2 443.76Control #3 298.41 Control #4 425.88 Mean Control 424.49 NanostructuredNitinol #1 961.61 Nanostructured Nitinol #2 1163.34 NanostructuredNitinol #3 2313 Nanostructured Nitinol #4 1364.52 Nanostructured Nitinol#5 1035 Mean Nanostructured Nitinol 1367.50

TABLE 3 Sample Total Cell Area (μm²) Control #1 1,056,118.14 Control #21,108,398.96 Control #3 737,071.23 Control #4 1,050,531.3 Mean Control98,8029.91 Nanostructured Nitinol #1 2,242,548.6 Nanostructured Nitinol#2 3,577,368.85 Nanostructured Nitinol #3 4,799,538 NanostructuredNitinol #4 2,982,154.35 Nanostructured Nitinol #5 2,487,768 MeanNanostructured Nitinol 3,217,875.56

Example 6. Smooth Muscle Cell Response to Oxide Nanostructures

Human aortic smooth muscle cells (HASMs) (passage 3) were cultured inwells with standard techniques on 500 um thick 4 mg/ml collagensubstrate (rat tail Corning #354236) at a seeding density of approx.120,481 cells per cm² until confluent (approx. 24 hours).

FIG. 17 is a schematic representation of the well setup 20. Asrepresented in FIG. 17 , an insert 30 (e.g. nanostructured nitinolcoupon, control coupon) was pushed into the collagen substrate 40 withHASMs 50, allowing the HASMs 50 to migrate onto the surface of theinsert 30. The well setup was then imaged along various focal planes 70,80, and 90.

Twenty 6 mm×6 mm×0.127 mm (W×H×D) nitinol foil coupons (Alfa Aesar)prepared as described in Example 3 above were cleaned three times eachwith 1 mL sterile PBS. The coupons were then inserted into 20 wells (onecoupon per well) with confluent HASM layers, as shown in FIG. 17 , withthe oxide nanostructures facing up (away from the confluent HAEC layer).20 untreated 6 mm×6 mm×0.127 mm (W×H×D) nitinol coupons (control) werethen cleaned three times each with 1 mL sterile PBS, and then insertedinto 20 wells (one coupon per well) with confluent HASM layers, as shownin FIG. 17 , with a clean native oxide layer facing up.

At time periods of 24 hours, 48 hours, 72 hours, and 96 hourspost-coupon insertion, wells were stained with CellTracker Green for 30minutes, fixed in 4% PFA, and counter-stained with Hoechst 33342, anuclear stain, for 20 minutes (five nanostructured coupons and 5 controlcoupons per time point). The wells were then imaged with ThermoScientific CX7 automated quantitative fluorescence microscope.Measurements were made to determine cell count, average cell area, andcell morphology (e.g. P2A, LWR). For purposes of these measurements,only cell nuclei associated with CellTracker Green were counted as livecells. No statistically significant differences were observed in cellcount or cell area after 96 hours (Tables 4 & 5, FIGS. 19A-D).

TABLE 4 Sample Total Nuclei Count Control #1 5267 Control #2 5633Control #3 6302 Control #4 4987 Control #5 5267 Mean Control 5491.2Nanostructured Nitinol #1 3347 Nanostructured Nitinol #2 5092Nanostructured Nitinol #3 5037 Nanostructured Nitinol #4 6735 MeanNanostructured Nitinol 5052.75

TABLE 5 Sample Total Cell Area (μm²) Control #1 4,037,261 Control #24,525,214 Control #3 4,652,515 Control #4 3,887,017 Control #5 4,230,718Mean Control 4,266,544.95 Nanostructured Nitinol #1 2,717,597Nanostructured Nitinol #2 4,085,923 Nanostructured Nitinol #3 4,202,671Nanostructured Nitinol #4 5,264,615 Mean Nanostructured Nitinol4,067,701

Example 7. Forming Metal Oxide Nanostructures on Nickel Titanium Foil

An electrolyte solution was prepared containing 0.8 vol % deionizedwater (H₂O) (e.g. 0.8 ml), 0.18 wt % ammonium fluoride (NH₄F) (e.g. 0.2g) (Sigma Aldrich), and 99.2 vol % ethylene glycol (e.g. 99.2 ml) (SigmaAldrich). The electrolyte solution was brought to and maintained at 30°C.

50 mm×50 mm×0.127 mm (W×H×D) nitinol foil (Alfa Aesar) was cut into 6mm×6 mm×0.127 mm (W×H×D) and 8 mm×8 mm×0.127 mm (W×H×D) coupons and thensuccessively ultrasonically cleaned with acetone, ethanol, and deionizedwater for 5 minutes each. The coupons were then kept in 70% ethanoluntil further use. Prior to anodization, the coupons were rinsed indeionized water and air dried.

For anodization, the cleaned nitinol coupons were then secured such thatonly one face of the coupon was exposed to the reaction conditions. Thesecured nitinol coupons were positioned approximately 2 cm from aplatinum cathode (Sigma Aldrich). The platinum cathode had the samesurface area as the nitinol coupon being anodized (e.g. 6 mm×6 mm or 8mm×8 mm).

A power supply (Agilent Technologies) provided a constant voltage of 25Vbetween the anode (nitinol coupon) and platinum cathode for 5 minutes.During this time, the current was variable as monitored by a digitalmultimeter (Agilent Technologies) connected to a desktop computerrunning BenchVue 3.1 (Keysight Technologies). A representative currentprofile (including the current from the second time period) is shown inFIG. 20 .

After the first 5 minute anodization, 56 mmol (10.8 g) citric acid(Sigma Aldrich) was added to the electrolyte solution from the firststep. The electrolyte solution continued to be maintained at 30° C. Thepower supply then provided a constant voltage of 25V between the anode(nitinol coupon) and cathode for another 5 minutes. During this second5-minute run, the electrolyte solution was stirred using magneticPTFE-coated stir bar at 300 rpm.

After the second 5-minute run, the coupons were rinsed in deionizedwater and kept in 70% ethanol until further use or evaluation. Thecoupons were then imaged using SEM (representative image shown in FIG.21 ).

Example 8. Forming Metal Oxide Nanostructures on Nickel Titanium Foil

An electrolyte solution was prepared containing 0.8 vol % deionizedwater (H₂O) (e.g. 0.8 ml), 0.18 wt % ammonium fluoride (NH₄F) (e.g. 0.2g) (Sigma Aldrich), and 99.2 vol % ethylene glycol (e.g. 99.2 ml) (SigmaAldrich). The electrolyte solution was brought to and maintained at 30°C.

50 mm×50 mm×0.127 mm (W×H×D) nitinol foil (Alfa Aesar) was cut into 6mm×6 mm×0.127 mm (W×H×D) and 8 mm×8 mm×0.127 mm (W×H×D) coupons and thensuccessively ultrasonically cleaned with acetone, ethanol, and deionizedwater for 5 minutes each. The coupons were then kept in 70% ethanoluntil further use. Prior to anodization, the coupons were rinsed indeionized water and air dried.

For anodization, the cleaned nitinol coupons were then secured such thatonly one face of the coupon was exposed to the reaction conditions. Thesecured nitinol coupons were positioned approximately 2 cm from aplatinum cathode (Sigma Aldrich). The platinum cathode had the samesurface area as the nitinol coupon being anodized (e.g. 6 mm×6 mm or 8mm×8 mm).

A power supply (Agilent Technologies) provided a constant voltage of 25Vbetween the anode (nitinol coupon) and platinum cathode for 5 minutes.During this time, the current was variable as monitored by a digitalmultimeter (Agilent Technologies) connected to a desktop computerrunning BenchVue 3.1 (Keysight Technologies). A representative currentprofile (including the current from the second time period) is shown inFIG. 22 .

After the first 5 minute anodization, 1.4 mmol (0.1162 ml) lactic acid(90%, Sigma Aldrich) was added to the electrolyte solution from thefirst step. The electrolyte solution continued to be maintained at 30°C. The power supply then provided a constant voltage of 25V between theanode (nitinol coupon) and cathode for another 5 minutes. During thissecond 5-minute run, the electrolyte solution was stirred using magneticPTFE-coated stir bar at 300 rpm.

After the second 5-minute run, the coupons were rinsed in deionizedwater and kept in 70% ethanol until further use or evaluation. Thecoupons were then imaged using SEM (representative image shown in FIG.23 ).

Additional Embodiments

In some embodiments, a metal oxide nanostructure formed on the surfaceof a substrate is provided, the metal oxide nanostructure comprising afirst layer comprising a plurality of tubular structures and a secondlayer comprising a lattice structure, wherein the first layer issituated between the substrate and the second layer.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the substrate comprises analloy of nickel and titanium.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the tubular structurescomprise titanium oxide and nickel oxide in a ratio greater than orequal to 1:1.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the plurality of tubularstructures are aligned generally perpendicular to the substrate andgenerally parallel to one another.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the plurality of tubularstructures each have an outer surface, and wherein the outer surface ofeach of the plurality of tubular structures are in contact with theouter surfaces of each of the adjacent tubular structures.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the lattice comprisestitanium oxide and nickel oxide.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the ratio of titanium oxideto nickel oxide on the distal surface of the lattice is greater than orequal to 10:1.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the substrate is animplantable medical device.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein the substrate is a stent.

Some embodiments include the metal oxide nanostructure of any one ormore of the preceding embodiments, wherein first and second layers areformed on a surface of the substrate using electrochemical anodization.

In some embodiments, a biocompatible surface formed on the surface of astent is provided, the biocompatible surface comprising a first layercomprising a plurality of tubular structures aligned generallyperpendicular to the surface of the stent and a second layer comprisinga metal oxide lattice defining openings therein, wherein the first layeris situated between the surface of the implantable medical device andthe second layer, the plurality of tubular structures each have an outersurface, the outer surface of each of the plurality of tubularstructures are in contact with the outer surfaces of each of theadjacent tubular structures, and a distal surface of the metal oxidelattice comprises titanium oxide and nickel oxide, with the ratio oftitanium oxide to nickel oxide being greater than or equal to 10:1.

Some embodiments include the biocompatible surface of any one or more ofthe preceding embodiments, wherein the surface promotes the growth ofhuman aortic endothelial cells at an interface between the stent and anartery.

Some embodiments include the biocompatible surface of any one or more ofthe preceding embodiments, wherein the surface inhibits the growth ofsmooth muscle cells at an interface between the stent and an artery.

In some embodiments, a method of forming a metal oxide nanostructure isprovided, the method comprising placing an anode and at least onecathode in electrical contact through a first electrolyte solutioncomprising an organic solvent, a fluoride-bearing species, and water,applying a first voltage across the anode and cathode through the firstelectrolyte solution for a first time period, modifying or replacing thefirst electrolyte solution resulting in a second electrolyte solutioncomprising an organic solvent, a fluoride-bearing species, water, and anacid, and applying a second voltage across the anode and cathode throughthe second electrolyte solution for a second time period.

Some embodiments include the method of forming a metal oxidenanostructure of any one or more of the preceding embodiments, furthercomprising providing an anode comprising nickel and titanium and alloysthereof and at least one cathode.

In some embodiments, a method of preparing a biocompatible surface isprovided, the method comprising placing an anode and at least onecathode in electrical contact through a first electrolyte solutioncomprising an organic solvent, a fluoride-bearing species, and water,applying a first voltage across the anode and cathode through the firstelectrolyte solution for a first time period, removing a portion of theanode surface, applying a second voltage across the anode and cathodethrough an electrolyte solution for a second time period.

Some embodiments include the method of preparing a biocompatible surfaceof any one or more of the preceding embodiments, further comprisingproviding an anode comprising nickel and titanium and alloys thereof andat least one cathode.

In some embodiments, a method of modifying the surface of an article isprovided, the method comprising anodizing at least a portion of asurface of an article in a first electrolyte solution, wherein the firstelectrolyte solution does not contain an acid, and anodizing the portionof the surface of the stent in a second electrolyte solution, whereinthe second electrolyte solution contains an acid.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding embodiments, wherein thearticle is anodized in the first electrolyte solution for about 5minutes at about 25 V.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding embodiments, wherein thearticle is anodized in the second electrolyte solution for about 5minutes at about 25 V.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding embodiments, wherein thearticle comprises nickel and titanium.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding embodiments, wherein thearticle is an implantable medical device.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding embodiments, wherein thearticle is a stent.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding embodiments, wherein theacid in the second electrolyte is hydroiodic acid.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding embodiments, wherein theacid in the second electrolyte is sulfuric acid.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding claims, wherein the acid inthe second electrolyte is lactic acid.

Some embodiments include the method of modifying the surface of anarticle of any one or more of the preceding claims, wherein the acid inthe second electrolyte is citric acid.

Any combination of methods, devices, systems, and features disclosedabove are within the scope of this disclosure.

Unless otherwise stated, all measurements, values, ratings, positions,magnitudes, sizes, and other specifications that are set forth in thisspecification, including in the claims that follow, are approximate, notexact. They are intended to have a reasonable range that is consistentwith the functions to which they relate and with what is customary inthe art to which they pertain.

All articles, patents, patent applications, and other publications thathave been cited in this disclosure are incorporated herein by reference.

In this disclosure, the indefinite article “a” and phrases “one or more”and “at least one” are synonymous and mean “at least one”.

The phrase “means for” when used in a claim is intended to and should beinterpreted to embrace the corresponding structures and materials thathave been described and their equivalents. Similarly, the phrase “stepfor” when used in a claim is intended to and should be interpreted toembrace the corresponding acts that have been described and theirequivalents. The absence of these phrases from a claim means that theclaim is not intended to and should not be interpreted to be limited tothese corresponding structures, materials, or acts, or to theirequivalents.

The scope of protection is limited solely by the claims that now follow.That scope is intended and should be interpreted to be as broad as isconsistent with the ordinary meaning of the language that is used in theclaims when interpreted in light of this specification and theprosecution history that follows, except where specific meanings havebeen set forth, and to encompass all structural and functionalequivalents.

Relational terms such as “first” and “second” and the like may be usedsolely to distinguish one entity or action from another, withoutnecessarily requiring or implying any actual relationship or orderbetween them. The terms “comprises,” “comprising,” and any othervariation thereof when used in connection with a list of elements in thespecification or claims are intended to indicate that the list is notexclusive and that other elements may be included. Similarly, an elementpreceded by an “a” or an “an” does not, without further constraints,preclude the existence of additional elements of the identical type.

None of the claims are intended to embrace subject matter that fails tosatisfy the requirement of Sections 101, 102, or 103 of the Patent Act,nor should they be interpreted in such a way. Any unintended coverage ofsuch subject matter is hereby disclaimed. Except as just stated in thisparagraph, nothing that has been stated or illustrated is intended orshould be interpreted to cause a dedication of any component, step,feature, object, benefit, advantage, or equivalent to the public,regardless of whether it is or is not recited in the claims.

While certain embodiments of the invention have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms. Furthermore, various omissions, substitutions and changes in thesystems and methods described herein may be made without departing fromthe spirit of the disclosure. The accompanying claims and theirequivalents are intended to cover such forms or modifications as wouldfall within the scope and spirit of the disclosure. Accordingly, thescope of the present inventions is defined only by reference to theappended claims.

Features, materials, characteristics, or groups described in conjunctionwith a particular aspect, embodiment, or example are to be understood tobe applicable to any other aspect, embodiment or example described inthis section or elsewhere in this specification unless incompatibletherewith. All of the features disclosed in this specification(including any accompanying claims, abstract and drawings), and/or allof the steps of any method or process so disclosed, may be combined inany combination, except combinations where at least some of suchfeatures and/or steps are mutually exclusive. The protection is notrestricted to the details of any foregoing embodiments. The protectionextends to any novel one, or any novel combination, of the featuresdisclosed in this specification (including any accompanying claims,abstract and drawings), or to any novel one, or any novel combination,of the steps of any method or process so disclosed.

Furthermore, certain features that are described in this disclosure inthe context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations, one or more features from a claimedcombination can, in some cases, be excised from the combination, and thecombination may be claimed as a subcombination or variation of asubcombination.

Moreover, while operations may be depicted in the drawings or describedin the specification in a particular order, such operations need not beperformed in the particular order shown or in sequential order, or thatall operations be performed, to achieve desirable results. Otheroperations that are not depicted or described can be incorporated in theexample methods and processes. For example, one or more additionaloperations can be performed before, after, simultaneously, or betweenany of the described operations. Further, the operations may berearranged or reordered in other implementations. Those skilled in theart will appreciate that in some embodiments, the actual steps taken inthe processes illustrated and/or disclosed may differ from those shownin the figures. Depending on the embodiment, certain of the stepsdescribed above may be removed, others may be added. Furthermore, thefeatures and attributes of the specific embodiments disclosed above maybe combined in different ways to form additional embodiments, all ofwhich fall within the scope of the present disclosure. Also, theseparation of various system components in the implementations describedabove should not be understood as requiring such separation in allimplementations, and it should be understood that the describedcomponents and systems can generally be integrated together in a singleproduct or packaged into multiple products.

For purposes of this disclosure, certain aspects, advantages, and novelfeatures are described herein. Not necessarily all such advantages maybe achieved in accordance with any particular embodiment. Thus, forexample, those skilled in the art will recognize that the disclosure maybe embodied or carried out in a manner that achieves one advantage or agroup of advantages as taught herein without necessarily achieving otheradvantages as may be taught or suggested herein.

Conditional language, such as “can,” “could,” “might,” or “may,” unlessspecifically stated otherwise, or otherwise understood within thecontext as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements, and/or steps. Thus, such conditional language is notgenerally intended to imply that features, elements, and/or steps are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or without userinput or prompting, whether these features, elements, and/or steps areincluded or are to be performed in any particular embodiment.

Conjunctive language such as the phrase “at least one of X, Y, and Z,”unless specifically stated otherwise, is otherwise understood with thecontext as used in general to convey that an item, term, etc. may beeither X, Y, or Z. Thus, such conjunctive language is not generallyintended to imply that certain embodiments require the presence of atleast one of X, at least one of Y, and at least one of Z.

Language of degree used herein, such as the terms “approximately,”“about,” “generally,” and “substantially” as used herein represent avalue, amount, or characteristic close to the stated value, amount, orcharacteristic that still performs a desired function or achieves adesired result. For example, the terms “approximately”, “about”,“generally,” and “substantially” may refer to an amount that is withinless than 10% of, within less than 5% of, within less than 1% of, withinless than 0.1% of, and within less than 0.01% of the stated amount. Asanother example, in certain embodiments, the terms “generally parallel”and “substantially parallel” refer to a value, amount, or characteristicthat departs from exactly parallel by less than or equal to 15 degrees,10 degrees, 5 degrees, 3 degrees, 1 degree, or 0.1 degree.

The scope of the present disclosure is not intended to be limited by thespecific disclosures of preferred embodiments in this section orelsewhere in this specification, and may be defined by claims aspresented in this section or elsewhere in this specification or aspresented in the future. The language of the claims is to be interpretedbroadly based on the language employed in the claims and not limited tothe examples described in the present specification or during theprosecution of the application, which examples are to be construed asnon-exclusive.

Of course, the foregoing description is that of certain features,aspects and advantages of the present invention, to which variouschanges and modifications can be made without departing from the spiritand scope of the present invention. Moreover, the devices describedherein need not feature all of the objects, advantages, features andaspects discussed above. Thus, for example, those of skill in the artwill recognize that the invention can be embodied or carried out in amanner that achieves or optimizes one advantage or a group of advantagesas taught herein without necessarily achieving other objects oradvantages as may be taught or suggested herein. In addition, while anumber of variations of the invention have been shown and described indetail, other modifications and methods of use, which are within thescope of this invention, will be readily apparent to those of skill inthe art based upon this disclosure. It is contemplated that variouscombinations or subcombinations of these specific features and aspectsof embodiments may be made and still fall within the scope of theinvention. Accordingly, it should be understood that various featuresand aspects of the disclosed embodiments can be combined with orsubstituted for one another in order to form varying modes of thediscussed devices.

The abstract is provided to help the reader quickly ascertain the natureof the technical disclosure. It is submitted with the understanding thatit will not be used to interpret or limit the scope or meaning of theclaims. In addition, various features in the foregoing detaileddescription are grouped together in various embodiments to streamlinethe disclosure. This method of disclosure should not be interpreted asrequiring claimed embodiments to require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus, the following claims are herebyincorporated into the detailed description, with each claim standing onits own as separately claimed subject matter.

The invention claimed may be:
 1. A biocompatible metal oxidenanostructure formed on a surface of a substrate, comprising: a firstlayer comprising a plurality of tubular structures; and a second layercomprising a lattice structure; wherein: the first layer is situatedbetween the substrate and the second layer; the substrate comprises analloy of nickel and titanium; and the atomic ratio of titanium to nickelon a distal surface of the second layer is greater than or equal to 10.2. The biocompatible metal oxide nanostructure of claim 1, wherein eachtubular structure is aligned substantially perpendicular to a surface ofthe substrate and substantially parallel to other tubular structures. 3.The biocompatible metal oxide nanostructure of claim 1, wherein eachtubular structure has an outer surface and wherein each tubularstructure's outer surface is in contact with the adjacent tubularstructure's outer surface.
 4. The biocompatible metal oxidenanostructure of claim 1, wherein the substrate is an implantablemedical device.
 5. The biocompatible metal oxide nanostructure of claim1, wherein the substrate is a stent.
 6. The biocompatible metal oxidenanostructure of claim 5, wherein the surface promotes the growth ofhuman aortic endothelial cells at an interface between the stent and anartery.
 7. The biocompatible metal oxide nanostructure of claim 1,wherein pitting corrosion is substantially absent from the substrate'ssurface.